Many diagnostic and surgical procedures utilizing indwelling medical devices require that the location of one or more fiducial markers or part(s) of an indwelling medical device be known with accuracy for the medical procedure to be properly performed and be successful. An example of the use of fiducial markers in a surgical context is the bracketing of a tissue volume containing a tissue mass, e.g., a non-palpable lesion, to be excised. Examples of this sort of tissue bracketing are disclosed in quite some detail in U.S. Pat. No. 6,698,433 to Dr. Krag, titled “System And Method For Bracketing And Removing Tissue,” which is incorporated herein by reference in its entirety. Tools for performing the bracketing method disclosed in Dr. Krag's patent include a locating device for locating each fiducial marker implanted prior to surgery. In order to locate the fiducial markers, the locating device must be as accurate as possible.
In Dr. Krag's tissue bracketing method, as well as in other in vivo methods utilizing fiducial markers, it is typically desired that the fiducial markers be as unobtrusive as practicable, while at the same time providing the necessary locational accuracy. Some conventional fiducial markers may be called “active” markers based on their direct excitation by electricity supplied to the markers via wires extending from the markers. Other conventional fiducial markers may be called “passive” markers in that they are not excited via hard-wiring, but rather are excited by radio frequency (RF) electromagnetic energy, emitted by a suitable transmitter. The fiducial markers utilized in Dr. Krag's tissue bracketing procedure are passive markers that contain resonance circuitry that responds to particular RF electromagnetic energy from the transmitter.
While these wired and resonant-type markers provide good locational accuracy, each has its drawback. An obvious drawback of wired markers is the presence of the wires, which can be obtrusive in many situations. A drawback of both wired and resonant-type markers is their microcircuitry, which makes the markers relatively complex and may require that the markers be larger than desirable. In addition, both types of markers require external energy sources that further add to the complexity of the systems utilizing such markers. It is highly desirable, therefore, that fiducial markers used for medical purposes be simple and capable of operating without any external energy source. This simplicity can translate into less complex systems for supporting the use of such markers.